Nitroxide Radicals as Templating Agents in the Synthesis of Magnets Based on Three-Dimensional Oxalato-Bridged Heterodimetallic Networks

Abstract

Building an extended compound with a particular crystal structure from molecular precursors in solution is still a challenge in many areas of chemistry. [1] Coordination chemistry provides a useful algorithm for this purpose (lattice engineering) because coordination geometries are well defined by strong and highly directional bonds. This concept is especially relevant in the synthesis of molecular magnetic materials since metal complexes provide the spin carriers (metal ions) and ligands through which magnetic interactions can occur. [2] The ability of the oxalate (ox) ion, C 2 O 4 2À , to transmit efficiently magnetic interactions through its bridging mode has been well documented since the pioneering work on dinuclear copper(ii) complexes. [3] Later, the use of the homoleptic species [M(ox) 3 ] 3À (M Cr III , Fe III ) provided an easy access to systems of higher dimensionality. [4] Only two families of high-dimensional oxalato complexes displaying magnetic order have been described: a) two-dimensional (2D) heterodimetallic compounds of formula A[M II M III (ox) 3 ] (A quaternary onium cation, M metal) with a honeycomb-layered structure [5] and b) three-dimensional (3D) homometallic compounds of formula A[M 2 II (ox) 3 ] or A[M II -M III (ox) 3 ](ClO 4 ) (A [M(bpy) 3 ] 2 ) with a cubic chiral pack-ing. [6] The resulting dimensionality depends on the type of A m ion used, so that these cations can be considered as templating agents for the overall structure. [7] There is a fine interplay between molecular recognition and chirality in this kind of system: 2D structures are achiral but, within each layer, all the M III sites have the same chirality while all the M II sites have the opposite one; in the 3D systems, both sites adopt the same configuration. In all cases, the enantioselective synthesis of the optically active stereoisomers using chiral building blocks is possible. [8] Besides their role as structuredirecting agents, the A m ions can also introduce physical properties of interest to the magnetic system, leading to multiproperty materials. Along this line, we have synthesized a family of 2D compounds where the ªinnocentº quaternary onium cation has been replaced by redox-active species of the decamethylmetallocenium type [9] or organic donors such as BEDT-TTF (2,2'-bis(5,6-dihydro-1,3-dithiolo[4,5-b][1,4]di-(50 MHz, CDCl 3 ): d